1. Field of the Invention
This invention relates to polycarbonates and more particularly to polycarbonates having improved heat distortion characteristics.
2. Description of the Prior Art
Polycarbonate resins derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded articles requiring impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties.
When a polycarbonate article is to be used in above ambient temperature environments, it is necessary that the polycarbonate be adapted to have proper physical properties at these temperatures. For example, polycarbonates have found extensive use in automotive applications due to their light weight in relation to their strength characteristics. When the polycarbonate is used in such applications, it must retain its dimensional stability during manufacture of the automobile. Particularly the polycarbonate must retain its dimensional stability in a paint oven or during similar high temperature processing of the vehicle.
Further, polycarbonates have been found useful in the appliance industry, for example in industrial power tool housings and similar applications wherein the tool, and hence the polycarbonate, is required to withstand continuous use at elevated temperatures.
All of these applications require dimensional stability at elevated temperatures.
Polycarbonates have been proposed and prepared which are useful at elevated temperatures. Exemplary of such proposed polycarbonates are those based upon fully fluorinated diphenolic compounds and optionally including such diols as 2,2'-bis(p-hydroxyphenyl) 1,1,1,3,3,3-hexafluoropropane as are disclosed in British Pat. No. 1,141,716 to Monsanto Chemicals, Ltd.
Accordingly, in order to provide this dimensional stability at elevated temperatures one would expect that an increase in molecular weight and hence a decrease in melt flow rate would necessarily improve the dimensional stability. However, this increase in molecular weight brings along with it a requirement that the processing temperature of the polycarbonate be increased in order to achieve the required processing. The increased processing temperature introduces problems such as thermal degradation and molding problems.
In accordance with the present invention, a polycarbonate is provided which has improved dimensional stability at elevated temperatures while retaining a relatively high melt flow rate for good processibility. Further, a polycarbonate prepared in accordance with the present invention can be synthesized in accordance with conventional techniques.